Zoned data communications system for communicating message signals between portable radios and a host computer

ABSTRACT

A data communications system is described that covers a geographic area divided into a plurality of non-overlapping zones and includes a general communications controller (GCC), a plurality of channel communications modules (CCM) and associated transmitters and receivers, and a plurality of portable radios. Message signals carrying alphanumeric information are communicated between the GCC and the portable radios by way of a radio channel. Each CCM takes a signal strength measurement every time it receives a message signal from a portable radio. The GCC gathers the signal strength measurements from the CCM receivers receiving the same message signal and computes an adjusted signal strength for each zone. The GCC then selects the zone having the largest adjusted signal strength for determining the location of the portable radio that transmitted the message signal. Whenever the GCC transmits a message signal to a portable radio, the CCM transmitter is used that covers the zone having the largest adjusted signal strength for the last transmission from that portable radio. Since the GCC can be simultaneously transmitting message signals to portable radios in other zones using non-interfering CCM transmitters, information throughput is greatly enhanced.

This is a division of application Ser. No. 441,327, filed Nov. 12, 1982,now U.S. Pat. No. 4,481,670.

BACKGROUND OF THE INVENTION

The present invention relates generally to radio communications systems,and more particularly to an improved method and apparatus fordynamically selecting one of a plurality of radio frequency signaltransmitters for transmitting message signals from a primary station toremote stations of a data communications system.

In radio communications systems covering large geographical areas, thelocation of remote stations such as portable or mobile radios must beknown with reasonable accuracy in order to provide good qualitycommunications. In such communications systems, the geographical areamay be divided up into a number of zones or cells, each of which iscovered by at least one radio transmitter and radio receiver associatedwith a primary station. For establishing communications between theprimary station and a remote station, knowledge of the remote station'slocation is necessary in order that the radio transmitter covering thezone in which the remote station is located may be selected at theprimary station.

The problem of selecting the radio transmitter which covers the zone inwhich a remote station is located has been solved with a limited degreeof success in several different ways. According to one technique, theradio receiver receiving the strongest RF signal from a selected remotestation is used to define the location of that remote station. Theprimary station simply selects the radio transmitter covering thegeographical area of the receiver receiving the strongest signal fromthe selected remote station.

According to another technique, each remote station is assigned to aspecific geographical area. In other words, a remote station ispermanently associated with the zone or zones covered by a specificradio transmitter. This technique works reasonably well as long as theremote station remains within the geographical area covered by theassigned radio transmitter. However, this technique is inadequate forcommunications systems where each remote station is free to move aboutthroughout a very large geographical area, making it impossible to limita remote station to the coverage area of a single radio transmitter.

According to yet another technique that is utilized in cellularradiotelephone systems, the remote station determines the zone in whichit is located by selecting the radio transmitter having the largestsignal strength. This technique requires that each radio transmitterhave a different frequency, and that communications from the primarystation to a selected remote station be sent in all zones in order forthe remote station to make its choice known on demand. This technique isadequate for radio telephone systems where the average message length ismuch longer than the minimum message length, but is inadequate for datacommunications sytems where the average message length is not muchlarger than the minimum message length. Therefore, in order to providegood quality communications in data communications systems, it isnecessary to have a reasonably accurate determination of the location ofeach remote station in the system.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved methodand apparatus for dynamically selecting one of a plurality of radiofrequency signal transmitters for transmitting message signals from aprimary station to a selected remote station of a data communicationssystem.

It is another object of the present invention to provide an improvedmethod and apparatus for dynamically determining the location of remotestations of a data communications system providing communicationsbetween a primary station and a plurality of remote stations locatedthroughout a large geographical area.

It is yet a further object of the present invention to provide animproved method and apparatus for simultaneously transmitting messagesignals to two or more remote stations located in different zones of adata communications system.

Briefly described, the present invention encompasses a datacommunications system for communicating message signals from a hostcomputer throughout a geographical area divided into zones, comprising:a communications controller coupled to the host computer forcommunicating message signals therebetween; a radio channel for carryingmessage signals; a plurality of remote radio stations located anywherein the geographical area and including a transmitter and antenna fortransmitting message signals on the radio channel and a receiverswithchably couplable to either the transmitter antenna or anotherantenna for receiving message signals; and a plurality of radio channelcommunications modules each located throughout the geographical area forcovering at least one zone and coupled to the communications controllerfor communicating message signals on the radio channel to the remoteradio stations in the zones covered thereby, each radio channelcommunications module coupled to a transmitter and antenna fortransmitting message signals on the radio channel and to a receiver andat least two antennas for receiving message signals from the radiochannel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a data communications system that mayadvantageously utilize the present invention.

FIG. 2 is a diagram of a geographical area that is divided up into anumber of zones.

FIG. 3 is a block diagram of the circuitry in the receivers in FIG. 1.

FIG. 4 is a block diagram of the circuitry in the channel communicationsmodules in FIG. 1.

FIG. 5 is a block diagram of the circuitry in the general communicationscontroller in FIG. 1.

FIG. 6 is a flow chart used by the general communications controller forprocessing signal strength data received from the channel communicationsmodules in FIG. 1.

FIG. 7 is a flow chart used by the general communications controller forselecting a transmitter on which data signals are transmitted to aselected portable radio in FIG. 1.

FIG. 8 is a flow chart used by the channel communications module formeasuring the signal strength of signals transmitted by the portableradios in FIG. 1.

FIG. 9 is a block diagram of the circuitry in the portable radios inFIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, there is illustrated a data communications system thatcommunicates message signals between a primary station, such as ageneral communications controller (GCC) 104, by way of a communicationsmedium, such as a radio frequency (RF) communications channel, to aplurality of remote stations, such as mobile or portable radios 130, 132and 134. Although described in the context of a data only communicationssystem, both data signals and analog signals such as voice signals canbe communicated over the RF communications channel to the portableradios 130, 132 and 134. The data communications system covers a largegeographical area which is divided into a plurality of cells or zones.Located throughout the geographical area are a number of channelcommunications modules (CCM) 106, 108, 110 and 112, which are eachcoupled to and control a number of RF signal transmitters 114, 120, and124 and RF signal receivers 116, 118, 122, 126 and 128.

The RF communications channel is preferably comprised of first andsecond carrier signals which may be modulated with the message signals.Transmitters 114, 120 and 124 may be operative on the first carriersignal, while receivers 116, 118, 122, 126 and 128 may be operative onthe second carrier signal of the radio communications channel. Each zoneof the radio communications system is covered by an assigned one of thetransmitters 114, 120 and 124 and by at least one of the receivers 116,118, 122, 126 and 128. Transmitters 114, 120 and 124 and receivers 116,118, 122, 126 and 128 may be any suitable commercially availabletransmitters and receivers such as those described in MotorolaInstruction Manual 68P81013E65. CCM's 106, 108, 110 and 112 may beco-located with their corresponding transmitters and receivers or may beremotely located and coupled to their corresponding transmitters andreceivers by means of a suitable remote control system, such as, forexample, the tone remote control system described in U.S. Pat. No.3,577,080.

Portable radios 130, 132 and 134 may be either commercially availablemobile radios of the type shown and described in Motorola instructionmanual no. 68P81039E25 or commercially available hand-held portableradios of the type shown and described in U.S. Pat. Nos. 3,906,166 and3,962,553 and in patent application Ser. No. 187,304 (now U.S. Pat. No.4,486,624), entitled "Microprocessor Controlled RadiotelephoneTransceiver", filed Sept. 15, 1980 and invented by Larry C. Puhl et al.Portable radios 130, 132 and 134 each include a transmitter operable onthe second carrier signal and a receiver operable on the first carriersignal. The transmitter and receiver in portable radios 130, 132 and 134may be any suitable commercially available conventional transmitter andreceiver, such as, for example, the transmitter and receiver describedin Motorola instruction manual no's. 68P81039E25 and 68P81014C65. Theseand the other Motorola Instruction Manuals referenced herein areavailable from the Service Publications Department of Motorola, Inc.,1301 East Algonquin Road, Schaumburg, Ill. or from Motorola C & E Parts,1313 East Algonquin Road, Schaumburg, Ill.

GCC 104 of the data communications system in FIG. 1 may be coupled to ahost computer 102 which may control a number of GCC's 104 that arelocated in different geographical areas, such as, for example, differentcities. Thus, host computer 102 may gather data from, and dispatch datato, portable radios located in several different cities. GCC 104 may becoupled to host computer 102 and CCM's 106, 108, 110, and 112 by meansof commercially available modems and associated dedicated telephonelines.

GCC 104 in FIG. 1 transmits message signals to and receives messagesignals from portable radios 130, 132 and 134. The message signals mayinclude coded data packets which each may contain a binary preamble, apredetermined synchronization word and an information word containing acommand, status or data. The format of the data packets may be any of anumber of existing data formats, such as, for example, those describedin U.S. Pat. Nos. 3,906,445, 4,156,867 and 4,354,252, and in U.S. patentapplication Ser. No. 402,682, entitled "Data Signalling System", filedJuly 28, 1982 and invented by Timothy M. Burke et. al.

Message signals are routed by GCC 104 to a selected CC'M 106, 108, 110and 112 for transmission by its corresponding transmitter. Since themessage signals are not transmitted on all transmitters simultaneously,as in simulcast systems of the type described in U.S. Pat. No.4,188,522, it is necessary that GCC 104 have a reasonably accuratedetermination of the location of each portable radio 130, 132 and 134 sothat GCC 104 may select the transmitter 114, 120 or 124 which covers thezone in which a particular portable radio is located. The improvedmethod and apparatus of the present invention enable GCC 104 todynamically select the transmitter 114, 120 or 124 for transmitting amessage signal to a selected portable radio 130, 132 or 134.

According to another important feature of the present invention, two ormore of the transmitters 114, 120 or 124 can be operated simultaneouslyfor communicating with different portable radios located in differentzones provided that transmissions from the two transmitters do notinterfere with reception in the particular zones where the two portableradios are located. As a result, data throughput of the datacommunications system illustrated in FIG. 1 can be significantlyincreased by re-use of the RF communications channel. In other words, bytaking advantage of re-use, a single RF communications channel can servethousands of portable radios in a geographical area covering severalstates and their major cities.

Referring to FIG. 2, there is illustrated a geographical area of a datacommunications system that is divided into seven zones, Z1-Z7, and thatincludes three CCM's 210,220 and 230 and corresponding transmitters andreceivers. Transmitter T1 of CCM 210 has a coverage area within circle212, transmitter T2 of CCM 220 within circle 222, and transmitter T3 ofCCM 230 within circle 232. Each time a portable radio transmits, signalstrength readings are taken by receivers R1, R2 and R3. These readingscan be expressed by the following signal strength SSI matrix:

    [SSI]=[SSI1 SSI2 SSI3].

According to the present invention, the signal strength readings takenby receivers R1, R2 and R3 can be used to compute an adjusted signalstrength for each zone Z1-Z7 by adjusting the measured signal strengthfor each receiver R1, R2 and R3 by corresponding predetermined factorsassociated with the particular zone and then combining the adjustedsignal strengths. The predetermined factors used to compute the adjustedsignal strength depend on a number of factors such as the terrain, theheight and gain of the antennas, and the sensitivity of the receivers.In other words, the predetermined factors associated with each zone areemperically determined and depend upon the characteristics of theequipment and terrain in each data communications system. Thepredetermined factors can be arranged in a zone selection ZSEL matrix,such as, or example, the exemplary ZSEL matrix hereinbelow: ##STR1## Theadjusted signal strength ZADJ matrix for each of the zones Z1-Z7 is thencomputed according to the following matrix formula:

    [ZADJ]=[SSI]×[ZSEL]; or

    [ZADJ]=[Z1ADJ Z2ADJ Z3ADJ Z4ADJ Z5ADJ Z6ADJ Z7ADJ]

Then, using the ZADJ matrix, GCC 104 can select the zone which has thelargest adjusted signal strength for a particular transmission from aportable radio. The selected zone can be stored together with other datain a location of the memory of GCC 104 associated with that portableradio.

Whenever transmitting a message signal to that particular portableradio, GCC 104 will first transmit the message signal on the carriersignal of the transmitter that covers the zone which had the largestadjusted signal strength for the last transmission from that portableradio. Both that zone and the transmitter covering it are stored in thememory of GCC 104. If the portable radio does not acknowledge thetransmission of the message signal from GCC 104, GCC 104 may attempt oneor more retransmissions of the message signal by means of that selectedtransmittter. If the retransmissions likewise are not acknowledged bythe portable radio, GCC 104 may then transmit the message signal via thetransmitter covering the zone which had the second largest adjustedsignal strength for the last transmission from that portable radio.Again, if the portable radio does not acknowledge the transmission fromGCC 104, GCC 104 may resend the message signal one or more times bymeans of that selected transmitter. If GCC 104 does not reach theselected portable radio by means of these two transmitters, GCC 104 mayeither select another transmitter covering that portable radio's "home"zone, or initiate a polling sequence in which the selected portableradio is polled in every zone in the data communications system startingwith the portable radio's "home" zone.

Assuming that the SSI matrix is 10, 10, 10 for a transmission from aselected portable radio, the ZADJ matrix will be 155, 153, 157, 207,202, 212, 226 using the predetermined factors in the above ZSEL matrix.For this particular transmission from that portable radio, the zonehaving the largest adjusted signal strength is zone Z7 and the zonehaving the second largest adjusted signal strength is zone Z6. Referringto FIG. 2, the portable station is most probably located in zone Z7which is approximately midway between CCM's 210, 220 and 230. The secondmost likely location of the portable station is zone Z6 which is betweenCCM's 220 and 230. The transmitters T1, T2 and T3 in FIG. 2 can beassigned to cover the seven zones as follows: Zone Z1 is covered by T1,zone Z2 is covered by T2, zone Z3 is covered by T3, zone Z4 is coveredby T1, zone Z5 is covered by T2, zone Z6 is covered by T3, and zone Z7is covered by T1. For transmitting a message signal to the portableradio, transmitter T1 is used first since zone Z7 has the largestadjusted signal strength. If the portable radio does not acknowledge thefirst transmission or subsequent re-transmissions from transmitter T1,the message signal is next transmitted by transmitter T3 for coveringzone Z6, which had the second largest adjusted signal strength for thelast transmission from the portable radio.

Assuming that on a subsequent transmission from the portable radio theSSI matrix is 10, 10, 0, the ZADJ matrix is 155, 153, 0, 107, 202, 102,152. In this case, zone Z5 has the largest adjusted signal strength, andzone Z1 has the second largest adjusted signal strength. Therefore, amessage signal would first be transmitted by transmitter T2 for coveringzone Z5, and thereafter transmitted by transmitter T1 for covering zoneZ1.

Again, assuming that a subsequent transmission from the portable stationresults in an SSI matrix that is 0, 10, 10, than the ZADJ matrix is 0,153, 157, 100, 98, 212, 149. In this case, zone Z6 has the largestadjusted signal strength, and zone Z3 has the second largest adjustedsignal strength. Since transmitter T3 covers both zone Z6 and zone Z3, amessage signal transmitted by transmitter T3 will reach the portableradio if it is in either zone Z6 or zone Z3. For a subsequenttransmission, zone Z2 has the third largest adjusted signal strength andis covered by transmitter T2.

Next, the transmitter re-use feature of the present invention may beillustrated by the seven zone arrangement in FIG. 2. First of all, thereis no transmitter interference for communications to portable radioslocated in zones Z1, Z2 or Z3. That is, transmitter T1, T2 and T3 can beoperated simultaneously for communicating with portable radios in zonesZ1, Z2 and Z3, respectively. However, for zone Z4, transmitter T3 mustbe off; for zone Z5 transmitter T1 must be off; for zone Z6 transmitterT2 must be off; and for zone Z7 transmitters T2 and T3 must be off.Using the foregoing interference criteria, transmitter re-use ispossible for all zones except for zone Z7. For example, if the portableradio is located in zone Z4, transmitter T1 is used to communicate withthat portable radio, and transmitter T2 can be simultaneously operatedfor communicating with portable radios in zone Z2. Similarly, whiletransmitter T3 is used for communicating with a portable radio in zoneZ6, transmitter T2 must be off and transmitter T1 can be on. In thiscase, transmitter T1 could be on and communicating with a portable radiolocated in zone Z1. Both a transmitter selection TSEL matrix and a zoneinterference ZIF matrix can be used to show the above criteria. The TSELmatrix is as follows:

    ______________________________________                                                   T1         T2    T3                                                ______________________________________                                        [TSEL] =   Z1    1            0   0                                                      Z2    0            1   0                                                      Z3    0            0   1                                                      Z4    1            0   0                                                      Z5    0            1   0                                                      Z6    0            0   1                                                      Z7    1            0   0                                           ______________________________________                                    

A one in the TSEL matrix indicates that the transmitter in that columnis used for communicating with a portable radio located in the zone inthat row.

The ZIF matrix is as follows:

    ______________________________________                                                  T1         T2    T3                                                 ______________________________________                                        [ZIF] =   Z1    0            0   0                                                      Z2    0            0   0                                                      Z3    0            0   0                                                      Z4    0            0   1                                                      Z5    1            0   0                                                      Z6    0            1   0                                                      Z7    0            1   1                                            ______________________________________                                    

A one in the ZIF matrix means that the transmitter in that column cannotbe transmitting if it is desired to communicate with a portable radiolocated in the zone in that row.

Both of these matrices can be provided by tables that are stored in thememory of GCC 104 in FIG. 1. GCC 104 uses both of this matrices duringthe process of selecting a transmitter for communicating a messagesignal to a selected portable radio. For example, assuming a portableradio is in zone Z5, transmitter T2 is used and transmitter T1 must beoff.

Referring to FIG. 3, there is illustrated a detailed circuit diagram ofthe receivers 116, 118, 122, 126 and 128 associated with CCM's 106, 108,110 and 112 in FIG. 1. Each receiver includes two antennas spaced at apredetermined distance from one another and a maximal ratio predetectiondiversity combiner 312, 314, 316, 318, 320, 322, 324, 326 and 328 forcombining the signals received by each of the antennas. The spacediversity provided by the two antennas is utilized to preventdegradation in communications which results when an antenna is locatedin an RF signal null. Rapid and deep RF signal nulls, called Rayleighfading, are experienced in communications systems operating at RF signalfrequencies in the new 800 to 900 mHz frequency range. The maximal ratiopredetection diversity combiner cophases the RF signals from eachantenna and linearly adds the cophased signals to provide a compositesignal having components that are proportional to the square of the RFsignals from each antenna. Therefore, strong signals are emphasized muchmore than weak signals. In other words, communications are not adverselyaffected if a very weak signal is received by one antenna and areasonably good signal is received by the other antenna.

In the diversity receiver in FIG. 3, the frequency of local oscillator306 determines the radio channel to which the diversity receiver istuned. The RF signal received by each antenna is combined by mixers 302and 304 with the signal from local oscillator 208 to providecorresponding IF signals. The IF signal from mixers 302 and 304 is thenapplied to IF bandpass filters 308 and 310, respectively, which may be amonolithic bandpass filter of conventional design similar to thatdescribed in U.S. Pat. No. 3,716,808. The filtered IF signals fromfilters 308 and 310 are split and fed forward via two paths to mixers312, 324 and 314, 326, respectively. First portions of the IF signalsare applied to mixers 324 and 326, and second portions of the IF signalsare applied to mixers 312 and 314 together with the composite IF signalwhich is fed back from amplifier 330. By feeding back the composite IFsignal, the IF strip of the diversity receiver forms a closed feedbackloop that is regenerative on noise. Thus, the randomly varying phase ofthe IF signals from filters 308 and 310 relative to the composite IFsignal is added into the closed loop via mixers 312 and 314 and thensubstracted out at mixers 324 and 326, respectively. By this process,the random phase variations are removed from the If signals in relationto the composite IF signal. The result is that each of the IF signals iscophased to the composite IF signal.

The product signals from mixers 312 and 314 at the difference frequencyare applied to filters 316 and 318, respectively, which each provide avariable phase shift. Filters 316 and 318 may be two-pole crystalfilters. The signals from filters 316 and 318 are linearly amplified byamplifiers 320 and 322, respectively, and applied to the second input ofmixers 324 and 326, respectively. Mixers 324 and 326 multiply thesignals from amplifiers 320 and 322, respectively, with the IF signalsfrom filters 308 and 310, respectively, to provide product signals thatare cophased with the composite IF signal. The product signals frommixers 324 and 326 are both cophased and proportional to the square ofthe level of the IF signals from filters 308 and 310, respectively. Theproduct signals from the mixers 324 and 326 are linearly added by summer328 to form one composite IF signal. The composite IF signal may becoupled via amplifier 330 to a conventional FM detector 332 which has anoutput signal providing demodulated message signals. The output signalof FM detector 332 is coupled to its corresponding CCM 106, 108, 110 or112 in FIG. 1. Further details of the circuitry in the diversityreceiver in FIG. 3 are illustrated and described in the instantassignee's co-pending U.S. patent applications, Ser. No. 22,757 (nowU.S. Pat. No. 4,369,520), filed on Mar. 22, 1979, entitled"Instantaneously Acquiring Sector Antenna System", and invented by FrankJ. Cerny, Jr. and James J. Mikulski, and in Ser. No. 268,613 filed onJune 1, 1981 (now abandoned), entitled "Large Dynamic Range Multiplierfor a Maximal Ratio Diversity Combiner", and invented by Frank J. Cerny,Jr.

FIG. 3 also illustrates the circuitry 340, 348 and 350 comprising thesignal strength detector that is located in the receivers. Summer 340 iscoupled to the signals from filters 308 and 310 and provides a compositesignal which is coupled to amplifier 348. The output of amplifier 348 iscoupled to envelope detector 350 which provides an SSI signal that isproportional to the maxima of the composite signal from amplifier 348. Aseparate amplifier 348 and envelope detector 350 can be provided foreach of the signals from filters 308 and 310 if it is desired to measureeach separately. The SSI signal from envelope detector 350 is coupled toits corresponding CCM 106, 108, 110 or 112 in FIG. 1, where it isdigitized. Many other types of commercially available signal strengthdetecting circuitry can be utilized in place of summer 340, amplifier348, and envelope detector 350.

Referring to FIG. 4, there is illustrated a block diagram of thecircuitry in CCM's 106, 108, 110 and 112 in FIG. 1. Each CCM includes amicrocomputer 402 having a memory with stored program therein forcommunicating with GCC 104 and portable radios 130, 132 and 134 inFIG. 1. Microcomputer 402 can be any suitable commercially availablemicrocomputer such as, for example, the Motorola type MC6800, MC6801 orMC68000 microprocessor, or those microprocessors described in U.S. Pat.Nos. 4,030,079 and 4,266,270, and the patents and patent applicationsreferred to therein.

Microcomputer 402 is coupled to RS232 interface 404 which may be coupledby a modem to a dedicated telephone line from GCC 104 in FIG. 1. Messagesignals received by microcomputer 402 from the GCC may be coupled tofilter 406 and thereafter applied to its corresponding transmitter. Themessage signals may be coded according to frequency-shift keying,phase-shift keying or any other suitable existing encoding scheme.Suitable message signal coding schemes are described in theaforementioned U.S. Pat. Nos. 3,906,445, 4,156,867 and 4,354,252 andpatent application Ser. No. 402,682. Message signals received fromportable radios by the CCM's receiver are coupled to filter 408 andthereafter to limiter 410 which converts the analog signals into anon-return-to-zero binary signal. The output of limiter 410 is appliedto an input port of microcomputer 402.

Microcomputer 402 also takes signal strength readings while it isreceiving message signals. The SSI signal from its correspondingreceiver is coupled to A/D converter 412, which may continuously convertthe analog SSI signal to a digitized SSI signal. The digitized SSIsignal from A/D converter 412 is applied to an input port ofmicrocomputer 402. Several A/D conversions are performed while a messagesignal is being received. The digitized SSI signals for the severalconversions are averaged by microcomputer 402. The average SSI signal isappended to the received message signal which is sent by microcomputer402 via RS232 interface 404 to GCC 104 in FIG. 1.

Referring to FIG. 5, there is illustrated a block diagram of thecircuitry in the general communications controller 104 in FIG. 1. TheGCC includes a microcomputer 500 having a memory with a stored programfor communicating with CCM's 106, 108, 110 and 112 in FIG. 1.Microcomputer 500 is coupled to RS232 interfaces 504, 505 and 506 whichmay be coupled by modems to dedicated telephone lines from each CCM.Microcomputer 500 is also coupled to RS232 interface 502 which may becoupled to a dedicated telephone line from host computer 102 in FIG. 1.Information in message signals received from portable radios by way ofCCM's 106, 108, 110 and 112 is forwarded by microcomputer 500 to hostcomputer 102. Conversely, information to be sent to portable radios fromhost computer 102 is transmitted to microcomputer 500 and incorporatedinto message signals transmitted to designated portable radios.Microcomputer 500 receives signal strength information from each of theCCM's whenever a portable radio transmits a message signal and processesthe signal strength information to determine the zone in which thatportable radio is presently located.

Microcomputer 500 stores for each portable radio the zone having thelargest adjusted signal strength for the last transmission, the zonehaving the second largest adjusted signal strength for the lasttransmission, the "home" zone assigned to that portable radio, and thelast zone used for communications with that portable radio. Forsubsequent transmissions of message signals to a portable radio, the GCCaccesses the zone location information for that portable radio andselects a transmitter for transmitting a message signal in the zone inwhich the portable radio is most likely located. Microcomputer 500 alsokeeps track of which transmitters are in use and which transmittersinterfere with communications in a particular zone. Thus, whentransmitting a message signal in the zone where a selected portableradio is located, microcomputer 500 inhibits the use of othertransmitters which would interfere with communications in that zone. Iftransmission of a message signal to a portable radio would intereferewith a transmission already under way, microcomputer 500 queues thatmessage signal for transmission when the interfering transmitter hascompleted its transmission. Microcomputer 500 can be any suitablecommercially available microcomputer, such as, for example, a Motorolatype MC6800, MC6801 or MC68000 microprocessor, or those microprocessorsdescribed in U.S. Pat. Nos. 4,030,079 and 4,266,270 and the patents andpatent applications referred to therein.

Referring next to FIG. 8, there is illustrated a flow chart includingthe process steps used by CCM's 106, 108, 110 and 112 in FIG. 1 formeasuring the signal strength of RF signals transmitted by portableradios. The flow chart in FIG. 8 provides a detailed description of theprocess steps required for execution by microcomputer 402 in FIG. 4. Thecoding of the process steps of the flow chart in FIG. 8 into theinstructions of a suitable commercially available microcomputer is amere mechanical step for a routineer skilled in the art.

Entering the flow chart in FIG. 8 at start block 800, a check is made tosee if the SSI flag is set at decision block 802. If the SSI flag is notset, NO branch is taken to decision block 820 where it is determinedwhether or not a SYNC (synchronization) word has been detected. The SYNCword is part of each data packet in a message signal and is followed byalphanumeric information. The particular bit pattern of the SYNC word isdetected by microcomputer 402 in FIG. 4. Signal strength measurementsneed not be taken until a SYNC word is detected. Once a SYNC word hasbeen detected, several signal strength measurements can be taken atdifferent times during receipt of the message signal and then averagedto obtain a more realistic estimate of the signal strength for theportable radio transmitting that message signal.

If a SYNC word has not been received, NO branch is taken from decisionblock 820 to block 822 to exit from the flow chart in FIG. 8. Otherwise,YES branch is taken from decision block 820 to block 824 where the SSIrunning average is cleared. Next, at block 826, the SSI flag is set, andthen at block 828 the SSI timer is set to twelve milliseconds. Assumingthat a data packet has a length of approximately twenty-fourmilliseconds, the SSI timer is set at twelve milliseconds so that twosignal strength measurements will be taken for each data packet. Next,the flow chart is exited at block 830.

Returning back to block 802 in FIG. 8, the SSI flag is set whenever amessage signal is being received from a portable radio. Assuming the SSIflag was previously set, YES branch is taken from decision block 802 todecision block 804 where it is determined if the SSI timer is equal tozero. Assuming that microcomputer 402 in FIG. 4 is interrupted onceevery millisecond, the SSI timer may be decremented and the flow chartin FIG. 8 may be executed every millisecond in response to eachinterrupt. As a result, the SSI timer will be zero twelve millisecondsafter a SYNC word has been received. If the SSI timer is not equal tozero, NO branch is taken to exit from the flow chart at block 806.Otherwise, YES branch is taken to block 808 where the digitized SSIsignal is read from A/D converter 412 in FIG. 4. Next, at block 810, thenewly read digitized SSI signal is averaged with the SSI runningaverage.

Proceeding to decision block 812 in FIG. 8, a check is made to determineif the end of the portable radio message signal has been reached. If theend of the message signal has not been reached, NO branch is taken toblock 828 where the SSI timer is set to twelve milliseconds for takinganother signal strength measurement. Otherwise, YES branch is taken fromdecision block 812 to block 814, where the SSI running average isappended to the message signal which is sent to GCC 104 in FIG. 1. Next,at block 816, the SSI flag is cleared in preparation for receipt ofsubsequent message signals, and the flow chart is exited at block 818.

The process steps of the flow chart in FIG. 8 are designed to take twosignal strength measurements for each data packet in a message signalreceived from a portable radio. For example, if there are four datapackets in a message signal, eight signal strength measurements aretaken and averaged. All CCM's 106, 108, 110 and 112 in FIG. 1 receivingthe same message signal from a portable radio are likewise taking twosignal strength measurements per data packet and appending the averagesignal strength to the message signal that is routed to the GCC.Therefore, within a short period of time, the GCC will be receivingseveral different average signal strength measurements from the CCM'sthat receive the same message signal from a portable radio.

Referring to FIG. 6, there is illustrated a flow chart used by GCC 104for processing the average signal strength measurements received fromeach of the CCM's 106, 108, 110 and 112 in FIG. 1. The flow chart inFIG. 6 is entered at start block 600 whenever a message signal togetherwith an average signal strength measurement is received from a CCM.Next, at block 602 a message timer is set to one-hundred milliseconds toprovide a time interval during which the same message signal is receivedby other CCM's and sent together with an average signal strengthmeasurement to the GCC. All CCM's should receive, if at all, the samemessage signal at approximately the same time. The one-hundredmillisecond message time interval is utilized to allow for CCMprocessing and transmission delays. Assuming that microcomputer 500 inFIG. 5 is interrupted once every millisecond, the message timer may bedecremented in response to each interrupt.

Next, at block 604 in FIG. 6, the average signal strength measurementreceived with a message signal is entered into the SSI matrix in theposition for the receiver that took the measurement. Proceeding todecision block 606, a check is made to see if another average signalstrength measurement has been received from another CCM. If so, YESbranch is taken back to block 604. Otherwise, NO branch is taken toblock 608 where the message timer is decremented once every millisecond.Next, at decision block 610 a check is made to see if the message timeris equal to zero. If not, NO branch is taken back to decision block 606to check to see if another average signal strength measurement has beenreceived. Otherwise, YES branch is taken to block 612 for processing theaverage signal strength measurements that have been received during theprevious one-hundred millisecond time interval.

Proceeding to block 612 in FIG. 6, an adjusted signal strength iscomputed for each zone using the newly received average signal strengthmeasurements that have been entered into the SSI matrix and thepredetermined factors previously entered into the ZSEL matrix. The ZADJmatrix is computed by multiplying the SSI matrix and the ZSEL matrixaccording to the formula:

    [ZADJ]=[SSI]×[ZSEL]

The resulting ZADJ matrix has one adjusted signal strength for each zonein the data communications system. Since some of the zones may be indifferent cities, some of the adjusted signal strengths may be zero. Forthe zone configuration in FIG. 2, it is possible that transmissions froma portable radio will be received by all three receivers R1, R2 and R3,producting an adjusted signal strength for all seven zones Z1-Z7.

According to another feature of the present invention, the SSI matrixcan be stored and later used in combination with the SSI matrix for thenext transmission from the same portable radio. For example, the signalstrength measurements in the stored SSI matrix can be decreased on thebasis of the time interval between the previous and newly receivedtransmission from the portable radio. Next, the decreased signalstrength measurements and the new signal strength measurements may beaveraged for each CCM receiver, and the average signal strengthmeasurements may be used to calculate the ZADJ matrix in block 612. Theupdated average signal strength measurements may then be stored in theSSI matrix for use with the signal strength measurements taken for asubsequent transmission from the same portable radio.

Next, at block 614 in FIG. 6, the zone having the largest adjustedsignal strength in the ZADJ matrix computed in block 612 is selected andstored in zone location Z(1) for the portable radio whose transmittedmessage signal was received by each of the CCM's. The number of CCM'sreceiving a message signal and making a signal strength measurement fora portable radio will vary depending both on the location of theportable radio and the terrain and location of receivers in thegeographical area of the data communications system. In other words,depending on the location of a portable radio, as few as one andpotentially all of the CCM receivers may receive the same message signalfrom a portable radio.

Next, at block 616, the zone having the second largest adjusted signalstrength in the ZADJ matrix is selected and stored in zone location Z(2)for the particular portable radio. Zone locations Z(1) and Z(2) are themost likely zones in which that portable radio is located. Every timethe portable radio transmits a message signal, new signal strengthmeasurements are taken and the zones stored in zone locations Z(1) andZ(2) are updated. Therefore, according to the present invention, thelocation of each portable radio is updated every time that portableradio transmits a message signal using the average signal strengthmeasurement taken by all of the CCM receivers that receive its messagesignal. Since the signal strength measurements from all CCM receiversreceiving the same message signal are used, a reasonably accuratedetermination of the portable radio's location can be made. To insurethat location information does not become stale, GCC in FIG. 1 caninitiate a short where-are-you message signal for those portable radiosthat have been inactive for a relatively long period of time.

Whenever it is desired to transmit a message signal from GCC 104 in FIG.1 to a selected portable radio, the flow chart in FIG. 7 is utilized bythe GCC for selecting the CCM transmitter covering the zone in which theselected portable radio is most likely to be located. Entering the flowchart in FIG. 7 at start block 700, N is set equal to 1 at block 702 andM is set equal to one at block 704. N is an integer number used todetermine which zone location Z(1), Z(2), Z(3) or Z(4) is selected, andM is an integer number used to determine the number of re-transmissionsmade to a particular zone.

Next, at block 706 in FIG. 7, the GCC selects the transmitter coveringzone location Z(N) for the selected portable radio. Initially, the GCCselects zone location Z(1). As previously explained, zone location Z(1)is the zone having the largest adjusted signal strength for the lasttransmission from the selected portable radio, zone location Z(2) is thezone having the second largest adjusted signal strength for the lasttransmission from the selected portable radio, zone location Z(3) is the"home" zone for the selected portable radio, and zone location Z(4) isthe zone location used for the last transmission to the selectedportable radio.

Proceeding next to decision block 708 in FIG. 7, a check is made to seeif an interfering transmitter is in use. The interfering transmittersare determined by reference to the ZIF matrix, which identifiestransmitters that interfere with communications in zone location Z(N).If an interfering transmitter is in use, YES branch is taken to block710 where the message signal is queued for later transmission to theselected portable radio, and the flow chart is exited at block 712. Ifan interfering transmitter is not in use, NO branch is taken to block714 where a message signal is transmitted to the selected portable radiousing a transmitter selected from the TSEL matrix for covering zonelocation Z(N). At the same time, interfering transmitters selected fromthe ZIF matrix for zone location Z(N) may be inhibited from transmittingwhile the message signal is being sent to the selected portable radio.

Next, at block 716 in FIG. 7, the GCC waits for one hundred millisecondsto determine if an acknowledgement message has been received from theselected portable radio. If the selected portable radio is actually inzone location Z(N) and receives the transmitted message signal, it willtransmit an acknowledgement signal indicating that the message signalhas been properly received. Proceeding to decision block 718, a check ismade to see if an acknowledgement signal has been received. If so, YESbranch is taken to block 720 and the flow chart is exited. In otherwords, the message signal has been successfully communicated to theselected portable radio. If an acknowledgement signal has not beenreceived, NO branch is taken to block 722 where M is incremented by 1.The variable M is used to provide for one or more re-transmissions ofthe message signal to the same zone location. In the preferredembodiment, one re-transmission is allowed. Therefore, at decision block724 a check is made to see if M is greater than or equal to three. If Mis less than three, NO branch is taken back to block 706 forre-transmitting the message signal to zone location Z(N). If M isgreater than or equal to three, YES branch is taken to block 726 forpreparing to transmit the message signal in the next zone location.

At block 726 in FIG. 7, N is incremented by one for selecting the nextzone location. Proceeding to decision block 728, a check is made to seeif N is greater than or equal to five. If N is less than five, NO branchis taken to block 704 where M is set equal to one and the process stepsare repeated for the next zone location Z(N). The process steps arerepeated beginning at block 704 for each of the zone locations Z(2),Z(3), and Z(4) so that a message signal is transmitted, andre-transmitted once, in all four stored zone locations in an attempt tocommunicate a message signal to a selected portable radio. If N isgreater than or equal to five, YES branch is taken from decision block728 to block 730 where the GCC alerts host computer 102 in FIG. 1 thatthe portable radio is either inactive or lost. At this point in time,the host computer may decide to poll the portable radio in every zone ofthe data communications system. Such a poll would be conducted on a lowpriority basis using a minimum length message signal. Next, the flowchart in FIG. 7 is exited at block 732.

The flow charts in FIGS. 6 and 7 provide a detailed description of theprocess steps used by GCC microcomputer 500 in FIG. 5 for communicatingmessage signals to portable radios. The coding of the process steps ofthe flow charts in FIGS. 6 and 7 into the instructions of a suitablecommercially available microcomputer is a mere mechanical step for aroutineer skilled in the art. By way of analogy to an electrical circuitdiagram, the flow charts in FIGS. 6, 7 and 8 are equivalent to adetailed schematic for an electrical circuit where provision of theexact part values for the electrical components in the electricalschematic corresponds to provision of microcomputer instructions forblocks in the flow charts.

Referring to FIG. 9, there is illustrated a block diagram of thecircuitry in portable radios 130, 132 and 134 in FIG. 1. Each portableradio includes a radio transceiver 340, a microcomputer 320, analphanumeric display 310, and a keyboard 312. Alphanumeric display 310may be any commercially available display, such as an LCD display or gasdischarge display, that provides for the display of one or more lines ofalphanumeric information. Display 310 is controlled by I/O device 321 ofmicrocomputer 320. Keyboard 312 may be any commercially availablekeyboard having both numeric and alphanumeric keys. Keyboard 312 iscoupled to I/O device 321 of microcomputer 320, which senses activationof its various keys.

Radio transceiver 340 in FIG. 9 may be any suitable commerciallyavailable transceiver, such as that described in the aforementionedMotorola instruction manual no. 68P81039E25 and in Motorola instructionmanual no. 68P81014C65. Radio transceiver 340 includes two antennasspaced at a predetermined distance from one another for providingreceiver diversity. Receiver 341 is coupled directly to one antenna andcoupled by duplexer 342 to the other antenna. Duplexer 342 may be anysuitable commercially available duplexer such as that described in U.S.Pat. No. 3,728,731. Receiver 341 may include suitable commerciallyavailable circuits for selecting between the two antennas, such as, forexample, the antenna selection circuitry in the aforementioned Motorolainstruction manual no. 68P81039E25. Receiver 341 demodulates messagesignals transmitted from the CCM transmitters. The demodulated messagesignals are filtered by filter 316 and limited by limiter 314 andthereafter applied to I/O device 321 of microcomputer 320. Messagesignals from I/O device 321 of microcomputer 320 are applied to filter318 and thereafter to transmitter 343 for transmission to CCM receivers.Transmitter 343 is turned on in response to the TX key signal from I/Odevice 321 of microcomputer 320. The output of transmitter 343 iscoupled to one of the radio transceiver antennas by way of duplexer 342.

Microcomputer 320 in FIG. 9 includes I/O devices 321, microprocessor(MPU) 322, random-access memory (RAM) 326, read-only memory (ROM) 323,and I.D. ROM 324. MPU 322 may be any suitable commercially availablemicroprocessor, such as, for example, the Motorola type MC6800, MC6801or MC68000 microprocessors, or those microprocessors described in U.S.Pat. Nos. 4,030,079 and 4,266,270 and the patent applications referredto therein. Similarly, I/O device 321, RAM 326, ROM 323 and I.D. ROM 324may be any commercially available devices that are suitable foroperation with the type of microprocessor selected for MPU 322. I.D. ROM324 is a removable device that includes a specific identification codeor address that is assigned to a portable radio. ROM 323 stores thecontrol program that is executed by MPU 322 for communicating messagesignals and acknowledgement signals to GCC 104 in FIG. 1. RAM 326includes both a scratch pad area used by MPU 322 during execution of thecontrol program stored in ROM 323 and a number of register locationsallocated for storing the identification code read in by MPU 322 from I.D. ROM 324, information displayed by display 310, information enteredfrom keyboard 312, and other status information. The contents ofspecific registers in RAM 326 may be loaded from message signalsreceived from GCC 104 in FIG. 1 or may be included in message signalssent by MPU 322 to the GCC. The formatting of register information intomessage signals may be accomplished as described in the aforementionedU.S. patent application, Ser. No. 402,682, which application alsoincludes a listing of suitable control program.

The portable radio illustrated in FIG. 9 may be either a mobile radiothat is installed in a vehicle or a portable radio that is small enoughto be hand-carried from place to place (See the aforementioned Motorolainstruction manual Number 68P81014C65). Although the portable radio inFIG. 9 is primarily adapted to transmit and receive message signalsincluding alphanumeric information, the portable radio may also providevoice communications by means of a speaker connected to the output ofreceiver 341 and a microphone connected to the input of transmitter 343.A portable radio adapted to communicate both alphanumeric informationand voice signals is described in the instant assignee's co-pending U.S.patent application Ser. No. 323,644, (now U.S. Pat. No. 4,430,792) filedNov. 20, 1981, entitled, "Data Muting Method and Apparatus for RadioCommunications System", and invented by Thomas A. Freeburg et. al.

In summary, unique methods and apparatus for transmitter selection andtransmitter re-use in data communications systems have been described.By selecting the proper transmitter for transmitting message signals toportable radios, unnecessary transmissions are eliminated, freeing upthe radio channel for communications with other portable radios.Moreover, transmitters which do not interfere with communicationsalready underway to a particular zone can be simultaneously transmittingmessage signals to portable radios in other zones, thus greatlyenhancing message signal throughput.

I claim:
 1. A data communications system for communicating messagesignals from a host computer throughout a geographical area divided intozones, comprising:a communications controller coupled to the hostcomputer for communicating message signals therebetween; a radio channelfor carrying message signals; a plurality of remote radio stationslocated anywhere in the geographical area and including a transmitterand antenna for transmitting message signals on the radio channel and areceiver switchably couplable to either the transmitter antenna oranother antenna for receiving message signals; and a plurality of radiochannel communications modules each located throughout the geographicalarea for covering at least one zone and coupled to the communicationscontroller for communicating message signals on the radio channel to theremote radio stations in the zones covered thereby, each radio channelcommunications module coupled to a transmitter and antenna fortransmitting message signals on the radio channel and to a receiver andat least two antennas for receiving message signals from the radiochannel.
 2. The data communications system according to claim 1, whereineach of said receivers coupled to a corresponding radio channelcommunications module further includes a maximal-ratio predetectiondiversity combiner coupled to the two antennas associated with thatreceiver for combining the signals received by each antenna to provide acomposite signal.